The present invention relates to hydraulic press brakes. More particularly, a method and apparatus for controlling the hydraulics of a press brake is provided.
Hydraulically driven press brakes are generally used for forming or bending metal. A press brake includes a stationary bed and a ram mounted above the bed. The ram is movable such that a workpiece placed between the ram and bed may be formed as the ram and the bed are brought together. The final shape of the workpiece is determined by the shape of the dies used in the forming process. Generally, a male die or a punch, is attached to the ram and a female die is attached to the bed.
Typically a hydraulic press brake has a rigid frame and hydraulic cylinders that drive the ram down towards the bed. A variety of mechanical methods used for attaching the male die or punch to the ram and the female die to the bed are known to those skilled in the art. A workpiece is formed into a shape as the ram is pushed down towards the bed. The hydraulic pressure in the cylinders supplies enough force so that the metal is formed into a shape that is governed by the opening of the female die.
When forming a workpiece, it is desired to repeatedly obtain the same shape from piece to piece. Repeatedly obtaining the same shape will help assure that the workpieces will perform the task for which they are intended. To obtain this piece to piece precision, several variables in the press brake must be controlled. For example, the method of controlling the position at which the downward motion of the punch is stopped will impact the shape of the workpiece that is formed. Thus, the downward position at which the punch is stopped in relation to the female die is an important variable in repeatedly obtaining a precise workpiece.
The ability to accurately stop the punch at the same position relative to the female die is highly dependent upon how fast the speed of the punch can be slowed to zero and how pressure from the output of a hydraulic pump is delivered to the cylinders. For example, a typical press brake contains a system of manifold valves or other flow bleed off mechanisms between the hydraulic pump and the press brake cylinders. When it is desired to stop the punch, hydraulic fluid from the pump is diverted or bypassed from the manifold valve system back to the hydraulic fluid reservoir. Thus, less fluid is directed to the cylinders and consequently the ram travels at a slower speed. One type of fluid bypass system is disclosed in U.S. Pat. No. 3,343,217 to Daubenberger. However, bypass systems do not provide precise and repeatable stopping positions because they often have response lag times that may vary from workpiece to workpiece.
Variable hydraulic pumps have also been used to control rams. For example, U.S. Pat. No. 2,396,296 to Stacey discloses a variable displacement pump that adjusts the pressure delivered to a ram. Further, U.S. Pat. No. 4,116,122 to Linder et al. discloses a hydraulic system in which the pump volume is controlled electronically through servo valves to vary the pump flow and the punch speed. This patent discloses utilizing a changing, stepped, ram speed to perform fine bending. Generally, servo valves require sophisticated microelectronic control and the accuracy of such valves is dependent upon the system hydraulic pressure level. Often such valves even utilize an external pressure source to operate properly.
It is desired to provide an alternative method to control the hydraulic pump output and the punch stopping point. It is desired to achieve a more repeatable punch stopping position so that workpieces may be formed more precisely with less variation between workpieces. It is also desired to provide quicker, smoother, and more complete ram speed deceleration to a zero speed. Furthermore, it is desired to achieve such objectives with a more economical system then servo valves and a system that is not as dependent upon the system hydraulic pressure level for fine accuracy. Finally, it is desired to achieve such objectives with a press brake that does not require stepped ram speeds.